Austenitic stainless steel sheet and a method for its manufacture

ABSTRACT

An austenitic stainless steel sheet for springs having both a high strength and excellent formability has a chemical composition comprising C: 0.01-0.15%, Si: at most 3.0%, Mn: at most 3.0%, Cr: 10.0-30.0%, Ni: 4.0-20.0%, N: at most 0.40%, and a remainder of Fe and impurities, and it has a metallurgical structure such that the austenite content γs (%) in the surface region of the steel sheet and the austenite content γc (%) in the center region of the sheet thickness satisfy (γs+γc)/2≦55 and γs/γc≧0.10, with the remaining structure being primarily strain-induced martensite.

TECHNICAL FIELD

This invention relates to an austenitic stainless steel sheet and amethod for its manufacture. More specifically, it relates to anaustenitic stainless steel sheet for use as a spring material havingboth a high strength and excellent formability and to a method for itsmanufacture.

BACKGROUND ART

Spring materials which are used in flat springs, spiral springs, frames,Belleville (coned disk) springs, dome switches, and the like forelectronic equipment, nuclear power facilities, automobile parts and thelike require having a high strength in order to reduce the thickness ofthe material and having excellent formability in order to fabricate itinto a desired product shape.

Materials which have thus far been used for these applications have beenSUS 301 (AISI 301) stainless steels which are classified as metastableaustenitic stainless steel. When a SUS 301 stainless steel undergoescold working, deformed portions transform into hard strain-inducedmartensite, thereby making it possible to obtain a high strengthrelatively easily. In addition, due to suppression of local deformationby the TRIP (transformation induced plasticity) effect, excellentformability can also be obtained. Such a spring material is disclosed inbelow-identified Patent Documents 1-3, for example.

Patent Document 1 discloses a stainless steel which has excellentformability and which contains C: at most 0.03% (in this description,unless otherwise specified, percent with respect to chemical compositionmeans mass percent), Si: at most 1.0%, Mn: at most 2.5%, Ni: 4.0-10.0%,Cr: 13.0-20.0%, N: 0.06-0.30%, S: at most 0.01%, and O: at most 0.007%,with the value of M=330−(480×C)−(2×Si)−(10×Mn)−(14×Ni)−(5.7×Cr)−(320×N)being at least 30.

Patent Document 2 discloses a stainless steel which has excellent springproperties and excellent fatigue properties in deformed portions andwhich contains C: at most 0.08%, Si: at most 3.0%, Mn: at most 4.0%, Ni:4.0-10.0%, Cr: 13.0-20.0%, N: 0.06-0.30%, and O: at most 0.007%, withthe value of above-described M being at least 40.

Patent Document 3 discloses a stainless steel which has excellentformability and fatigue properties and which contains C: at most 0.03%,Si: greater than 1.0% to at most 3.0%, Mn: at most 4.0%, Ni: 4.0-10.0%,Cr: 13.0-20.0%, N: at most 0.30%, S: at most 0.01%, and O: at most0.007%, with the value of above-described M being in the range of30-100.

All of the stainless steels disclosed in Patent Documents 1-3 aremanufactured by carrying out cold rolling with a rolling reduction of atleast 50% after hot rolling, then carrying out finish annealing at arelatively low temperature for a relatively short time so as to producerefined, uniform recrystallized grains with an average grain diameter ofat most 10 μm, and finally performing temper rolling. Namely, in each ofthese stainless steels, it is attempted to obtain desired strengthproperties by utilizing grain refinement, which is a strengtheningmechanism accompanied by little deterioration in formability. However,in recent years, the strength demanded of spring materials isincreasing, and the stainless is steels disclosed in Patent Documents1-3 sometimes do not have sufficient strength demanded of products.

Below-identified Patent Document 4 discloses a high strength springmaterial based on a low-C, high-N SUS 301L steel and specifically astainless steel having a mixed phase structure comprising at least 40%by area of martensite and a remainder of austenite or a single-phasemartensitic structure which is obtained by temper rolling with areduction of at least 30% of a stainless steel having a chemicalcomposition containing C: at most 0.03%, Si: at most 1.0%, Mn: at most2.0%, Cr: 16.0-18.0%, Ni: 6.0-8.0%, N: at most 0.25%, and Nb: 0-0.30%and a structure in which the percent area of recrystallized grainshaving an average grain diameter of at most 5 μm is at least 50% andless than 100% and the percent area of unrecrystallized portions isgreater than 0% and at most 50%.

With the stainless steel disclosed in Patent Document 4, after ametallurgical structure including a strain-induced martensite is formedby temper rolling, the steel is formed into a predetermined shape andthen subjected to aging treatment, thereby causing fine chromiumnitrides to precipitate in martensite. By utilizing the precipitationstrengthening at this time, it is possible to achieve a high strengthwithout adding an additional step.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP 04-214841 A-   Patent Document 2: JP 05-279802 A-   Patent Document 3: JP 05-117813 A-   Patent Document 4: JP 4,321,066 B

SUMMARY OF THE INVENTION

In order to decrease the size and improve the performance of products,there is today a demand for spring materials having a higher strengthand better formability. Therefore, even the stainless steel disclosed inPatent Document 4 is sometimes unable to fully satisfy the performancedemanded of the newest products.

According to one aspect, the present invention is an austeniticstainless steel sheet characterized by having a chemical compositioncomprising C: 0.01-0.15%, Si: at most 3.0%, Mn: at most 3.0%, Cr:10.0-30.0%, Ni: 4.0-20.0%, N: at most 0.40%, and a remainder of Fe andimpurities, and by having a metallurgical structure in which theaustenite content γs (%) in a surface region of the steel sheet and theaustenite content γc (%) in a center region of the thickness of thesteel sheet satisfy (γs+γc)/2≦55 and γs/γc≧0.10, with the remainingstructure being primarily strain-induced martensite.

The austenite content γs (%) in a surface region of the steel sheetmeans the volume percent of austenite contained in a region from theoutermost surface of the steel sheet to a depth of 10 μm in the sheetthickness direction (this region being referred to as the surface regionof the steel sheet). The austenite content γc (%) in a center region ofthe sheet thickness means the volume percent of austenite contained in aregion at a depth of 10 μm in the sheet thickness direction from asurface which is formed by removing one-half of the original sheetthickness by mechanical grinding and chemical polishing of one side ofthe steel sheet (this region being referred to as the center region ofthe steel sheet).

The chemical composition of an austenitic stainless steel shv eetaccording to the present invention may further contain, in place of aportion of Fe,

-   -   (1) at least one of Mo: at most 3.0% and Cu: at most 3.0%,        and/or    -   (2) at least one of Ti: at most 0.50%, Nb: at most 0.50%, and V:        at most 1.0%.

From another aspect, the present invention is a method of manufacturingan austenitic stainless steel sheet characterized by hot rolling a steelhaving the above-described chemical composition, then carrying out coldrolling and annealing on the resulting hot rolled steel sheet to obtainan annealed cold rolled steel sheet, and then subjecting the annealedcold rolled steel sheet to temper rolling with the number of passesbeing at least the rolling reduction (%)/10.

In the above-described method, the average grain diameter of austenitegrains in the annealed cold rolled steel sheet before temper rolling ispreferably at most 5 μm.

The present invention provides an austenitic stainless steel sheethaving both a high strength and excellent formability and a method forits manufacture.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is an explanatory view showing an example of processing steps ofan austenitic stainless steel sheet according to the present inventionafter temper rolling;

FIG. 2 is an explanatory view showing an example of the relationshipbetween the distribution in the sheet thickness direction of theaustenite content after temper rolling and formability.

FIG. 3 is an explanatory view showing a method of evaluatingformability.

MODES FOR CARRYING OUT THE INVENTION

The present invention will be explained in greater detail whilereferring to the attached drawings.

An austenitic stainless steel sheet according to the present inventionis a cold rolled steel sheet which has undergone temper rolling. Asshown in FIG. 1, the austenitic stainless steel sheet is subjected,after temper rolling, to forming in order to impart a desired shape andthen, if necessary, to aging treatment, when products (such as varioustypes of springs) are fabricated therefrom.

The present inventors came up with the idea that a strain-inducedmartensitic structure formed by temper rolling could be strengthened byincreasing the content of C, which is a solid solution strengtheningelement, thereby making it possible to achieve an increase in strengthof steel. The above-described problem of insufficient strength can besolved by the combination of strengthening of martensite phases byincreasing the C content and precipitation strengthening utilizing Cr₂N.

To this end, if the amount of strain-induced martensite is low and alarge amount of austenite remains, a desired strength cannot beobtained. Therefore, the average of the austenite content γs (%) in thesurface region of the steel sheet and the austenite content γc (%) inthe center region of the sheet thickness, namely, (γs+γc)/2 (below, thisvalue will be referred to as the average austenite content) is made atmost 55. “γs” and “γc” are defined as set forth above.

The present inventors also came up with the idea that controlling thedistribution of the austenite content in the sheet thickness directionis effective at improving the formability of a steel sheet, whichdecreases as strength increases. FIG. 2 is an explanatory view showingan example of the relationship between the distribution in the sheetthickness direction of the austenite content after temper rolling andformability.

As shown in FIG. 2, even when the average austenite content after temperrolling is the same, it is possible to greatly improve the formabilityof the temper-rolled steel sheet by varying the distribution in thesheet thickness direction of the austenite content after temper rolling.Specifically, by increasing the austenite content remaining in thesurface region of a steel sheet after temper rolling, strain-inducedmartensitic transformation takes place sufficiently to adequately inducethe TRIP effect in the surface region of the steel sheet, which is mostdeformed in subsequent forming operations. As a result, excellentformability is obtained.

In addition, if this austenitic stainless steel sheet is then treated byaging, fine Cr₂N precipitates mainly in the martensite phases having alow solubility for N during aging treatment, thereby making it possibleto also utilize the effect of age strengthening. In this manner, anaustenitic stainless steel sheet according to the present invention canexhibit both a high strength and excellent formability.

The heat generated by working within a steel sheet during temper rollingincreases as the amount of reduction per pass in temper rollingincreases. Therefore, the temperature of the surface of a steel sheet,which is cooled with a rolling oil, becomes markedly lower than thetemperature of the center region of the thickness of the sheet, andduring the next pass of rolling, the amount of martensite which forms inthe surface region of the steel sheet markedly increases, leading to asignificant decrease in the austenite content in the surface region ofthe steel sheet.

Namely, if temper rolling is carried out with a small number of passesas in a conventional manner, the austenite content remaining in thesurface region of the steel sheet becomes markedly smaller than theaustenite content remaining in the center region of the thickness of thesheet. During subsequent forming, the TRIP effect resulting fromstrain-induced martensitic transformation of austenite is not adequatelyobtained, and formability decreases.

In contrast, if the number of passes in temper rolling is increased andthe rolling reduction per pass is decreased to suppress the generationof heat during working, the amount of austenite remaining in the surfaceregion of the steel sheet after temper rolling can be increased. As aresult, a distribution of the austenite content in the sheet thicknessdirection which is desirable for subsequent forming can be achieved.

Specifically, if the distribution of the austenite content in the sheetthickness direction of a steel sheet after temper rolling is such thatthe austenite content γs (%) in the surface region of the steel sheetand the austenite content γc (%) in the center region of the sheetthickness satisfy the condition γs/γc≧0.10, during the subsequentforming operations, the TRIP effect is sufficiently exhibited in thesurface region of the steel sheet, which is most greatly deformed in theforming operations.

Even if the number of passes in temper rolling is increased so as todecrease the rolling reduction in each pass, the temperature of thesurface region of the steel sheet becomes lower than that of the centerregion of the sheet thickness. As a result, it cannot be avoided that alarger amount of martensite is formed in the surface region of the steelsheet than in the center region of the sheet thickness, thereby makingthe austenite content in the surface region lower than that in thecenter region. However, it was found that sufficient formability foractual use is guaranteed if the austenite content in the surface regionof the steel sheet is at least 1/10 of the austenite content in thecenter region of the sheet thickness.

As explained above, the present invention is based on the technicalconcept of “achieving a great increase in strength by the combination ofstrengthening of martensite phases by an increased C content andprecipitation strengthening with Cr₂N, and at the same time achievingexcellent formability by optimizing the distribution of the austenitecontent in the sheet thickness direction, whereby an austeniticstainless steel sheet can be obtained which satisfies the demand for amaterial which can form spring parts having decreased size and weight.”

Next, the chemical composition, the metallurgical structure, and amanufacturing method for an austenitic stainless steel sheet accordingto the present invention will be explained.

(1) Chemical Composition

C: 0.01-0.15%

C is a solid solution strengthening element, and it is extremelyeffective at strengthening a martensite phase which is formed duringcold working. Therefore, is the C content is made at least 0.01%.However, if the C content is excessive, coarse carbides are formedduring manufacturing process of steel sheets, and formability andcorrosion resistance deteriorate, so the C content is made at most0.15%. The C content is preferably at least 0.03%.

Si: at most 3.0%

Si is a solid solution strengthening element, thereby imparting a highstrength to steel, and it is also used as a deoxidizing agent at thetime of melt refining. However, if the Si content is excessive, coarseSi compounds are formed during manufacturing process of steel sheets,and these coarse Si compounds lead to a deterioration in hot workabilityand cold workability. Therefore, the Si content is at most 3.0% andpreferably at most 2.8%.

Mn: at most 3.0%

Mn is used as a deoxidizing agent at the time of melt refining. Inaddition, Mn is an austenite stabilizing element, and it is contained ina suitable amount taking into consideration the balance with otherelements. However, if the Mn content is excessive, coarse Mn compoundsare formed during manufacturing process of steel sheets, and the coarseMn compounds become the starting points of fractures, causingformability to deteriorate. Therefore, the Mn content is at most 3.0%and preferably at most 2.8%.

Cr: 10.0-30.0%

Cr is a basic element in stainless steel. When its content is at least10.0%, it forms a passive film on the surface of steel and provides theeffect of increasing corrosion resistance. Furthermore, when steel issubjected to aging treatment, it contributes to an increase in thestrength of steel by precipitating as fine Cr₂N. However, Cr is aferrite-forming element, so if the Cr content is excessive, delta (δ)ferrite forms at high temperatures and the hot workability of steel ismarkedly decreased. Therefore, the Cr content is at least 10.0% and atmost 30.0%, and preferably at least 12.0% and at most 25.0%.

Ni: 4.0-20.0%

Ni is a basic element in an austenitic stainless steel. At least 4.0% ofNi is contained in order to stably obtain an austenite phase having anexcellent balance between strength and ductility at room temperature.However, if the Ni content is excessive, an austenite phase becomes toostable and the occurrence of strain-induced martensitic transformationis suppressed, and a high strength cannot be obtained. Therefore, the Nicontent is at least 4.0% and at most 20.0%, and preferably at least 4.5%and at most 18.0%.

N: at most 0.40%

Like C, N is a solid solution strengthening element which contributes toincreasing the strength of steel. Furthermore, when steel undergoesaging treatment, it contributes to increasing the strength of steel byprecipitating as fine Cr₂N. However, if the N content is too high, theoccurrence of edge cracks is easily induced at the time of hot working.Therefore, the N content is at most 0.40% and is preferably at least0.05% and at most 0.30%.

An austenitic stainless steel sheet according to the present inventionmay further contain the following optional elements as necessary.

One or both of Mo: at most 3.0% and Cu: at most 3.0%

Mo and Cu are both elements which precipitate as fine intermetalliccompounds during aging treatment and thereby contribute to increasingthe strength of a steel sheet, so they may be added if necessary.However, if the Mo content or the Cu content is excessive, delta (δ)ferrite may easily form at high temperatures and precipitate at grainboundaries, resulting in a marked deterioration in hot workability.Therefore, the Mo content and the Cu content are each at most 3.0% andpreferably each at most 2.8%.

At least one of Ti: at most 0.5%, Nb: at most 0.5%, and V: at most 1.0%

Ti, Nb, and V each precipitate as fine carbides or nitrides duringmanufacturing process of steel sheets, thereby suppressing the growth ofcrystal grains by the pinning effect and contributing to increasing thestrength of a steel sheet by precipitation strengthening. Therefore,they may be contained if necessary. However, if the content of theseelements is excessive, they form coarse carbides or nitrides whichbecome starting points of fracture at the time of deformation andmarkedly worsen formability. Therefore, the Ti content and the Nbcontent are made at most 0.5%, and the V content is made at most 1.0%.Preferably, the Ti content and the Nb content are at most 0.4%, and theV content is at most 0.8%.

The remainder other than the above-described elements is Fe andimpurities. Examples of typical impurities are P: at most 0.05% and S:at most 0.03%.

(2) Metallurgical Structure

[Austenite Distribution in the Sheet Thickness Direction]

As a result of carrying out a variety of tests, the present inventorsfound that when the austenite content γs (%) in the surface region of asteel sheet and the austenite content γc (%) in the center region of thesheet thickness of the steel sheet satisfy the following Equations (1)and (2) and the remainder of the structure is constituted primarily by astrain-induced martensitic structure, an austenitic stainless steelsheet having both a high strength and formability is obtained:

(γs+γc)/2≦55  Equation (1)

γs/γc≧0.10.  Equation (2)

By making the average austenite content, which is the average of theaustenite content γs in the surface region of the steel sheet and theaustenite content γc in the center region of the sheet thickness, atmost 55% as shown by Equation (1) and making the remainder primarilyhigh strength strain-induced martensite, a high strength steel isobtained. The average austenite content is preferably at most 50%, morepreferably at most 45%, still more preferably at most 40%, and mostpreferably at most 35%. There is no particular lower limit on theaverage austenite content, but if the austenite content is extremelysmall, a sufficient TRIP effect may not be obtained in the surface ofthe steel sheet at the time of forming, so it is preferably at least 5%and more preferably at least 7.5%.

By making the ratio (γs/γc) of the austenite content γs in the surfaceregion of the steel sheet to the austenite content γc in the centerregion of the sheet thickness at least 0.10 as shown by Equation (2),the TRIP effect resulting from the strain-induced martensitictransformation of austenite is adequately exhibited even in the surfaceof a steel sheet, which undergoes the greatest deformation at the timeof forming of a sheet, and excellent formability is obtained. The ratioγs/γc is preferably at least 0.2, more preferably at least 0.3, stillmore preferably at least 0.5, and most preferably at least 0.6.

In the present invention, a high strength and excellent formability canboth be achieved by having the austenite content in the surface regionof the steel sheet and the austenite content in the center region of thesheet thickness satisfy Equation (1) and Equation (2).

The remainder of the metallurgical structure other than austenite isprimarily a strain-induced martensite phase. This strain-inducedmartensite is formed by temper rolling of a steel sheet which has beenannealed after cold rolling. Therefore, an austenitic stainless steelsheet according to the present invention is a temper rolled material.

“Primarily a strain-induced martensite phase” means that strain-inducedmartensite is at least 50 volume % of the remainder of the structureother than austenite. In an austenitic stainless steel sheet which ismanufactured in accordance with the below-described method according tothe present invention, the metallurgical structure consistssubstantially of austenite and strain-induced martensite. Examples ofother phases are fine precipitates (carbides, nitrides, andcarbonitrides), but the amount of these phases is extremely small. Asingle-phase martensitic structure in which γs=γc=100% is outside thescope of the present invention.

As stated above, the austenite content is larger in the center region ofthe sheet thickness than in the surface region of the steel sheet.Therefore, even in an austenitic stainless steel sheet according to thepresent invention having an increased austenite content in the surfaceregion of the steel sheet, the relationship γs<γc (namely γs/γc<1) issatisfied.

[Grain diameter of austenite grains before temper rolling: at most 5 μm]

Refinement of crystal grains is known as a method of strengthening steelwith little deterioration in ductility. This is also an effectivestrengthening method in the stainless steel which is the object of thepresent invention. In addition, by decreasing the grain diameter andincreasing the density of grain boundaries, strains which concentrate atgrain boundaries at the time of forming are dispersed, thereby achievingthe effect of suppressing the occurrence of cracks. In the presentinvention, the grain diameter of austenite grains in a steel sheetbefore temper rolling (an annealed cold rolled material) is preferablyat most 5 μm.

(3) Manufacturing Method

In accordance with the present invention, the above-described austeniticstainless steel sheet according to the present invention can bemanufactured by carrying out hot rolling of a steel material having theabove-described chemical is composition, then subjecting the resultinghot rolled steel sheet to cold rolling and annealing to obtain anannealed cold rolled steel sheet, and subjecting the annealed coldrolled steel sheet to temper rolling with the number of passes being atleast the rolling reduction (%)/10.

Hot rolling, cold rolling, and annealing may each be carried out in aconventional manner. Cold rolling is preferably performed one to threetimes so that the overall rolling reduction is around 30-90%, and aftera predetermined overall reduction has been obtained, annealing is thencarried out. It is also possible to repeat multiple passes of coldrolling and annealing. There is no particular limit on the number ofpasses of cold rolling which are carried out prior to temper rolling.

It is preferable to make the overall reduction in cold rolling largeenough to obtain a refined metallurgical structure in which the averagegrain diameter of austenite grains in the annealed cold rolled steelsheet which is subjected to the subsequent temper rolling is at most 5μm, because particularly formability is improved thereby.

[Temper Rolling Conditions]

In the present invention, temper rolling is carried out in a strongmanner in order to make the most use of strengthening achieved bystrain-induced martensitic transformation. The overall reduction oftemper rolling is preferably at least 40%, more preferably at least 50%,and most preferably at least 60%. There is no particular upper limit onthe overall reduction of temper rolling, but normally it is less than100% and preferably at most 90%.

If such strong temper rolling is carried out with a small number ofpasses, as stated above, strain-induced martensitic transformation ispromoted in the surface region of the steel sheet, and the austenitecontent in this region is so decreased that it is no longer possible tosatisfy the condition that the ratio of the austenite content γs in thesurface region of the steel sheet to the austenite content γc in thecenter region of the sheet thickness (γc/γs) is at least 0.1, leading toa deterioration in formability.

As a result of investigating the relationship between the number ofpasses in temper rolling and the austenite distribution in the thicknessdirection of a steel sheet, the present inventors confirmed that bycarrying out temper rolling such that is the number of passes is atleast the overall reduction (%)/10 as shown by Equation (3), the ratioγc/γs becomes at least 0.10. Therefore, temper rolling is carried outsuch that the number of passes is at least the overall reduction intemper rolling (%) divided by ten (10). For example, when the overallreduction in temper rolling is 65%, the number of passes is made atleast 7.

Number of passes in temper rolling≧Overall reduction in temperrolling(%)divided by 10.  Equation (3)

Preferably, the reduction in each pass of temper rolling is nearly thesame. Accordingly, the reduction in each pass of temper rolling ispreferably made at most 10%. Since excessively increasing the number ofpasses worsens productivity, the number of passes is preferably in therange from the smallest number of passes satisfying the total reduction(%)/10 to 2 passes larger than the smallest number.

Example 1

Table 1 shows the chemical compositions of stainless steels used in thisexample. Steels A-F are inventive steels which satisfy the compositiondefined by the present invention, and steels G-M are comparative steelswhich do not satisfy the composition defined by the present invention.

Table 2 shows the manufacturing conditions and the test results forsteel sheets manufactured using steels A-M. Steel sheets 1-8 areinventive steel sheets which satisfy the requirements of the presentinvention, while steel sheets 9-18 are comparative steel sheets which donot satisfy the requirements of the present invention.

Steels having the chemical compositions shown in Table 1 were melted ina conventional atmospheric melting furnace to obtain 17-kg ingots. Eachof these ingots was subjected to hot rolling and annealing to obtain ahot rolled steel sheet with a thickness of 6.0 mm, and then this hotrolled steel sheet underwent cold to rolling and annealing one to threetimes to obtain an annealed cold rolled steel sheet having a thicknessof 0.8-4.0 mm. This annealed cold rolled steel sheet was subjected to aplurality of passes of temper rolling to finally obtain a thin sheetwith a thickness of 0.4 mm. Temper rolling was carried out underconditions such that the reduction in each pass was the same.

Using test pieces which were taken from the steel sheets before andafter the temper rolling, the grain size, the austenite content,formability, and tensile strength were investigated by the followingmethods. Some of the steel sheets were subjected to aging treatment at300° C. for one minute after temper rolling. The tensile strength ofthese steel sheets was the value after aging treatment.

(Average Grain Diameter After Annealing)

The grain diameter of austenite grains was determined as the nominalgrain diameter of austenite grains in a scanning electronphotomicrograph of an etched cross section of a test piece taken fromthe annealed cold rolled steel sheet before temper rolling.

(Austenite Content)

The austenite content was determined for the surface region of a testpiece taken from a steel sheet after temper rolling and for the surfaceof the center region of the sheet thickness which was exposed bymechanical grinding and chemical polishing. The austenite content wasdetermined using the integrated intensity ratio obtained by X-raydiffraction measurement and a scanning electron photomicrograph afteretching. In Table 2, the austenite content in the surface region of asteel sheet is indicated by γs, and the austenite content in the surfaceof the center region of the sheet thickness is indicated by γc.

(Formability)

FIG. 3 is an explanatory view showing a method for evaluatingformability. A 100 mm-square test piece taken from a steel sheet aftertemper rolling was subjected to shallow draw forming as shown in FIG. 3.Thereafter, the edge of the hole was examined for cracks under anoptical microscope. A case in which no cracks were ascertained at allwas evaluated as DOUBLE CIRCLE (excellent), a case in which continuouscracks were not ascertained was evaluated as CIRCLE (o, acceptable), anda case in which continuous cracks were ascertained or in which fractureoccurred was evaluated as X (unacceptable).

(Tensile Strength)

Tensile strength was measured in accordance with JIS Z 2241 using a JISNo. 13B tensile test piece taken from a steel sheet after temper rollingor after aging treatment. The measured value is shown together with aCIRCLE (acceptable) for specimens having a tensile strength exceeding1500 N/mm² and with an X (unacceptable) for specimens which did notreach this level.

TABLE 1 Mark C N Cr Ni Si Mn Mo Cu Nb Ti V Inven- A 0.049 0.12 17.1 6.90.48 2.48 0.5 0.5 0 0 0 tive B 0.050 0.12 17.2 6.9 2.51 0.51 0.5 0.50.05 0.05 0.05 C 0.121 0.08 19.1 5.1 0.55 2.01 0.5 0.5 0 0 0 D 0.0500.15 17.0 6.9 0.54 0.51 0 0 0 0 0 E 0.052 0.12 12.9 10.0 0.51 0.45 0.50.5 0 0 0 F 0.082 0.1 13.8 4.9 0.64 0.78 0.01 0.05 0 0 0 Compar- G 0.1700.41 12.5 5.5 0.48 0.48 0.5 0.5 0 0 0 ative H 0.008 0.03 14.9 7.2 0.552.1 0.5 0.5 0 0 0 I 0.031 0.05 31.3 21.1 1.02 1.03 0.2 0.2 0 0 0 J 0.0300.05 7.8 3.9 1.03 0.99 0.2 0.2 0 0 0 K 0.049 0.1 18.0 8.1 3.51 3.39 0.50.5 0 0 0 L 0.052 0.12 17.1 7.1 0.55 2.51 3.5 3.2 0 0 0 M 0.032 0.0517.0 6.9 0.52 0.48 0.5 0.5 0 1.0 0 Note) The underlined values areoutside the range defined by the present invention.

TABLE 2 Manufacturing conditions Anneal- % number Grain dia- Testresults ing reduction of passes % Aging meter after Tensile Steel Steeltemp. in temper in temper reduction/ temp. annealing γs γc (γs + γc)/2Form- strength sheet mark (° C.) rolling rolling 10 (° C.) (μm) (%) (%)(%) γs/γc ability (N/mm²) Inven- 1 A 950 70 8 7 300 10.1 21.5 27.5 24.50.78 ◯ ◯1792 tive 2 A 950 70 7 7 10.0 20.0 28.0 24.0 0.71 ◯ ◯1712 3 A850 70 7 7 300 4.0 25.0 34.5 29.8 0.72 ⊚ ◯1827 4 B 900 60 6 6 300 4.824.0 45.0 34.5 0.53 ⊚ ◯1856 5 C 950 50 5 5 300 9.8 33.0 54.0 43.5 0.61 ◯◯1658 6 D 950 70 7 7 300 8.5 14.5 48.5 31.5 0.30 ◯ ◯1712 7 E 950 90 10 9 300 9.8 9.5 11.5 10.5 0.83 ◯ ◯1922 8 F 950 80 10  8 9.1 5.5 9.5  7.50.58 ◯ ◯1903 Compar- 9 A 950 90 5 9 300 10.1 2.0 27.5 14.8 0.07 X ◯1908ative 10 E 950 90 3 9 300 9.8 1.0 11.0  6.0 0.09 X ◯1945 11 F 950 80 3 89.0 0.5 8.5  4.5 0.06 X ◯1906 12 G 950 70 7 7 300 11.2 54.5 62.0 58.30.88 X ◯1552 13 H 950 50 2 5 300 9.5 5.5 64.5 35.0 0.09 X X1458 14 I 95060 6 6 300 11.3 89.0 97.0 93.0 0.92 ◯ X1022 15 J 950 50 2 5 300 9.3 2.529.5 16.0 0.08 X ◯1786 16 K 950 50 5 5 300 10.8 68.0 92.0 80.0 0.74 XX1150 17 L 950 70 7 7 300 11.0 54.8 81.5 68.2 0.67 X X1332 18 M 950 70 35 300 9.5 3.5 38.0 20.8 0.09 X ◯1703 Note) Underlinesd values or marksare outside the range defined by the present invention.

Steel sheets 1-8 in Table 2, which were steel sheets according to thepresent invention, had excellent formability and a high strength.Comparing steel sheets 1 and 2, it was ascertained that a particularlyhigh strength was obtained by precipitation of fine Cr₂N by agingtreatment. In addition, it was ascertained that steel sheets 3 and 4,which had a grain diameter of at most 5 μm after annealing, had aparticularly high strength and excellent formability.

Steel sheets 9-18 were comparative examples for which the chemicalcomposition or the manufacturing conditions were outside the rangedefined by the present invention.

Steel sheets 9-11 had a γs/γc ratio of less than 0.1, so a high strengthwas obtained but formability was poor. Comparing steel sheet 7 withsteel sheet 10 or steel sheet 8 with steel sheet 11, although steelsheets 7 and 8 had both a high strength and formability, steel sheets 10and 11 had a high strength but poor formability. Therefore, it wasascertained that even if steels with the same chemical composition aremanufactured with the same reduction in temper rolling, the distributionof the austenite content varies and properties greatly vary with thenumber of rolling passes in temper rolling.

For steel sheet 12, the C content and the N content were above the rangefor the present invention. As a result, coarse carbonitrides wereformed, and formability markedly worsened.

Steel sheet 13 had a Cr content below the range for the presentinvention, so it had a low strength after aging treatment. In addition,γs/γc was less than 0.1, so formability was poor.

For steel sheet 14, the Cr content and the Ni content were above therange for the present invention, and the average values of γs and γcexceeded 55. Therefore, its strength was low even after aging treatment.

For steel sheet 15, the Cr content and the Ni content were below therange for the present invention and γs/γc was less than 0.1, soformability was poor.

For steel sheet 16, the Si content and the Mn content were above therange for the present invention, and the average values of γs and γcexceeded 55. Therefore, the strength was low even after aging treatment.In addition, coarse Si compounds and Mn compounds formed, so formabilitywas also poor.

For steel sheet 17, the Mo content and the Cu content were above therange for the present invention, and the average values of γs and γcexceeded 55, so the strength was low even after aging treatment. Inaddition, coarse intermetallic compounds were formed, and formabilitywas also poor.

For steel sheet 18, the Ti content was above the range for the presentinvention, so coarse TiN formed and formability was poor.

1. An austenitic stainless steel sheet characterized by having achemical composition comprising, in mass %, C: 0.01-0.15%, Si: at most3.0%, Mn: at most 3.0%, Cr: 10.0-30.0%, Ni: 4.0-20.0%, N: at most 0.40%,and a remainder of Fe and impurities, and by having a metallurgicalstructure in which the austenite content γs (%) in a surface region ofthe steel sheet and the austenite content γc (%) in a center region ofthe sheet thickness satisfy (γs+γc)/2≦55 and γs/γc≧0.10, with theremaining structure being primarily strain-induced martensite.
 2. Anaustenitic stainless steel sheet as set forth in claim 1 wherein thechemical composition contains, in mass %, at least one of Mo: at most3.0% and Cu: at most 3.0% in place of a portion of Fe.
 3. An austeniticstainless steel sheet as set forth in claim 1 wherein the chemicalcomposition contains, in mass %, at least one of Ti: at most 0.50%, Nb:at most 0.50%, and V: at most 1.0% in place of a portion of Fe.
 4. Amethod for manufacturing an austenitic stainless steel sheet as setforth in claim 1 characterized by performing hot rolling of a steelhaving the above-described chemical composition, then performing coldrolling and annealing of the resulting hot rolled steel sheet to obtainan annealed cold rolled steel sheet, and subjecting the annealed coldrolled steel sheet to temper rolling with the number of passes being atleast the reduction (%)/10.
 5. A method as set forth in claim 4 whereinthe average grain diameter of the austenite grains of the annealed coldrolled steel sheet before temper rolling is at most 5 μm.
 6. Anaustenitic stainless steel sheet as set forth in claim 2 wherein thechemical composition contains, in mass %, at least one of Ti: at most0.50%, Nb: at most 0.50%, and V: at most 1.0% in place of a portion ofFe.
 7. A method for manufacturing an austenitic stainless steel sheet asset forth in claim 2 characterized by performing hot rolling of a steelhaving the above-described chemical composition, then performing coldrolling and annealing of the resulting hot rolled steel sheet to obtainan annealed cold rolled steel sheet, and subjecting the annealed coldrolled steel sheet to temper rolling with the number of passes being atleast the reduction (%)/10.
 8. A method for manufacturing an austeniticstainless steel sheet as set forth in claim 3 characterized byperforming hot rolling of a steel having the above-described chemicalcomposition, then performing cold rolling and annealing of the resultinghot rolled steel sheet to obtain an annealed cold rolled steel sheet,and subjecting the annealed cold rolled steel sheet to temper rollingwith the number of passes being at least the reduction (%)/10.
 9. Amethod for manufacturing an austenitic stainless steel sheet as setforth in claim 6 characterized by performing hot rolling of a steelhaving the above-described chemical composition, then performing coldrolling and annealing of the resulting hot rolled steel sheet to obtainan annealed cold rolled steel sheet, and subjecting the annealed coldrolled steel sheet to temper rolling with the number of passes being atleast the reduction (%)/10.